The present invention relates to improvements in centrifuge separating systems, and more particularly to an improved solid drum and worm centrifuge for the separation of sludges into a solid phase and a liquid phase, and particularly sludges from the biological clarification stage of a sewage treatment plant where the sludge has a capability of generating a biogas, and a prime mover system for driving the centrifuge.
Centrifuge separating systems are employed universally for sludge dewatering or sludge thickening of sludges. An example of this type of operation is in the treatment of municipal sludges in sewage treatment plants. Solid drum or bowl centrifuges which have an advancing screw or worm therein are utilized to dewater or separate thin sludges low in solids as well as thick slurries high in solids and these systems are used widely in sewage treatment plants. With a solid bowl worm centrifuge of the type described, the solids particles are settled or separated onto the inner surface of the centrifuge drum by the action of centrifugal force and these are moved of discharged by the conveyor screw which rotates independently inside the rotating drum with a differential speed which is either slower or faster, that is, it leads or lags in comparison to the rotational speed of the drum.
It has heretofore been the practice in driving the centrifuge of a solid bowl worm separator used in sewage treatment plants by an electric motor. Normally, this is a three phase AC motor or a DC motor.
It is also known in the driving of a solid bowl worm centrifuge that the electric motor drives the centrifuge through a hydraulic drive mechanism which incorporates a hydraulic pump delivering its liquid to a hydraulic motor which is in driving connection with the separator. Normally, the drive will be connected to both the conveyor screw and the drum with a gearing arrangement such as a planetary gear or a controllable speed differential gearing arrangement interconnecting the drum and screw to obtain the desired speed differential.
In the separation process utilizing sewage sludge, gas is generated and this gas is eliminated by burning. The gas essentially consists of methane and is conveniently referred to as biogas which develops in the digestion tower of a sewage treatment plant as a result of digestion or fermentation of the sludge resulting from the biological clarification stage. Motors operative on this biogas have been used to drive generators and the electrical output of the generators has been used to power electrical motors which have been used to drive the centrifuges and the supply of electrical current has resulted partially from the biogas generators as well as outside sources. Such a drive system for a centrifuge requires high capital costs as well as high operating costs because the overall efficiency of the drive system cannot be high due to the number of energy conversions necessary from the beginning of the chain to the driving of the centrifuge. On the other hand, systems wherein biogas is produced from the biomass on the basis of microbiological and biochemical processes and wherein thin sludges or thick slurries must be dewatered or thickened are increasing in number.
It is, therefore, an object of the present invention to provide a centrifuge system utilizing a solid bowl worm centrifuge for the separation of solids or liquids from sludges which will be more effective and more efficient than heretofore available.
In accordance with the principles of the invention, a system is utilized wherein the drive of the centrifuge is provided by an essentially direct gas motor drive operated from biogas as a fuel.
Various advantages are achieved by the invention including the fact that wherein the centrifuge is driven directly by a biogas operated motor utilizing a hydraulic pump and a hydraulic motor, these elements normally already exist in systems provided and the electric drive motor and current generator for the electric motor are eliminated, this effecting a considerable savings of capital cost. The specific energy consumption or, in other words, the operating cost of the present system are considerably lower due to the reduction of energy conversion stages in the drive system and due also to the increase in overall efficiency connected with a better energy exploitation of the biogas which results from the anaerobic sludge treatment of a sewage treatment plant. Due to the direct employment of the biogas arising in a sewage treatment plant or similar system which generates biogas for the direct drive of the sludge centrifuge, energy self-sufficiency of the system is realized or at least noticeably increased. The more efficient exploitation of the energy contained in the biogas such as in sewage treatment plants becomes more important in view of the fact that systems for the treatment of sewage are becoming more sophisticated out of necessity due to the tightening of environmental protection laws and an increasing quantity of sewage sludge and thus of biogas resulting therefrom. In the event of a shortage of biogas, the gas motor serving as the drive for the centrifuge can be operated with alternative fuel such as natural gas, propane and the like. The waste heat of the biogas motor which is removed by means of its coolant and exhaust gases can likewise be utilized.
It is another feature of the present invention to provide a solid bowl worm centrifuge which can be driven by a single biogas operated motor via a hydraulic drive which comprises a hydraulic pump and two hydraulic motors which can be separately driven via respective dual biogas motors. Self-employment of biogas arising in the sewage treatment plant for the drive of the sludge centrifuge provides a further feature in that the biogas motor is connected by a gasline to a biogas tank which connects the biogas of a digestive sludge tower of the clarification plant where the sludge is being thickened or dewatered. The biogas motor can advantageously be disposed separated from the centrifuge such as in a sound insulated room. The base frame of the centrifuge then will no longer carry the drive motor and the weight of the centrifuge and its vibrations are thereby reduced. According to a further feature of the invention, the centrifuge with the hydraulic pump and motor, but without the biogas motor can be mounted on an undercarriage and thus be movable to various places of employment. The possibility is also provided for the biogas motor to drive a further electric generator in addition to the hydraulic pump of the hydraulic drive. This means that with an excess of biogas which is not needed for driving the centrifuge, this excess can be utilized for supplying additional power such as to consumers in the area of the sewage treatment plants.
Other objects, advantages and features of the invention will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: